Conventional spring-loaded electrical contact probes generally include an outer receptacle, a barrel containing a movable plunger and a spring, which exerts a force against the tail of the plunger to bias the plunger outwardly from the barrel. The plunger may be depressed inwardly into the barrel a predetermined distance, under force directed against the spring.
Battery-type contacts and interconnect probes generally require compact, durable, highly reliable designs with circuit paths optimized for the best performance. These contacts and probes are typically employed in battery charging applications, mobile telecommunication applications, docking applications, and other portable electronic devices in addition to applications for testing electronics, printed circuit boards and computer chips, for example. They may be used as either power conductors or as signal carriers and are subject to a wide range of environmental conditions.
As products continue to shrink in size or increase in performance while maintaining current size, the need for smaller contacts continues to grow. The resultant probe often performs well mechanically but the electrical performance in certain instances is compromised by the action of the spring and device under test. Specifically, if the device under test pushes directly down on top of the plunger and the spring generates a force pushing directly up the desired contact between plunger and barrel, which is required for optimal electrical performance, can be very light or nonexistent. The result is a poor, unreliable electrical performance for the probe.
As is known in the art, current travels in parallel down all available paths in a quantity dependent upon the path's resistance. A common spring, by nature of its design and composition, has a very large resistance and causes poor-electrical performance of the probe if it is the main circuit path. Likewise, large resistances between the barrel inner diameter (“ID”) and plunger, referred to as the internal contact resistance, may also lead to poor electrical performance or failure of the probe. Large internal contact resistances are generally due to low contact force between barrel ID and plunger, poor conductive material of barrel and plunger including plating material and contaminates such as dirt, lint, or lubricants. To improve electrical performance of the probe, designs minimize the internal contact resistance by proper material selection, plating selection, attention to cleanliness/handling, and increasing the contact force between the barrel ID and plunger through efforts called biasing. Biasing is the action of forcing the plunger's bearing surface against the barrel ID.
In an effort to improve biasing in probes many designs have been generated. One design utilizes a bias cut on the tail of the plunger where the tail of the plunger is cut at an angle. A large side force is created from the spring pushing against the bias cut creating firm, constant contact force between barrel and plunger. This contact force ensures that the current will flow from the plunger to the barrel and not through the spring and also provides the lowest contact resistance between barrel and plunger. The disadvantage to this type of design is the higher friction that is created between plunger and barrel resulting in failure of the probe due to mechanical wear. Another problem is often the pointing accuracy of the probe suffers with a bias cut design. The pointing accuracy of a probe is the maximum radial departure of a probe tip from the center line of the probe barrel. With a bias cut, the plunger tail is forced to one side of the barrel which results in the tip being forced away from the center line of the probe. Additionally, as the probe wears, the pointing accuracy may degrade.